The invention relates to electrochromic compounds capable of attenuating the transmittance of the near infrared portion of the electromagnetic spectrum and, more particularly to electrochromic devices comprising an electrochromic medium that has at least one electrochromic compound capable of reversibly attenuating the transmittance of the near infrared portion of sunlight.
Electrochromic windows have been proposed for many years to attenuate the amount of sunlight that is transmitted into a building. It would be advantageous during the summer months to decrease the total amount of solar energy entering a building, and increase the total amount of solar energy entering a building during the winter months. This would provide a substantial energy savings for interior space heating and air-conditioning. C. G. Granqvist states, in the "Handbook of Inorganic Electrochromic Materials", Elsevier N.Y. (1995), that WO.sub.3 in its crystalline and highly doped form exhibits a change in reflectance in the near-infrared portion of the electromagnetic spectrum. Several organic polymers show changes in absorbance in the near-infrared range. In the discussions herein, near infrared, or "NIR" is defined as electromagnetic radiation in the range of about 750-2400 nm. Most commercial electrochromic systems have been designed to attenuate only the visible portion of the solar spectrum. Since solar energy is, on the average, 7.9% ultraviolet (UV), 45.5% visible radiation, and 46.7% near-infrared (NIR) radiation, over one-half of the total solar energy is not in the visible portion of the spectrum.
In U.S. Pat. No. 4,902,108, entitled "Single-Compartment, Self-Erasing, Solution-Phase Electrochromic Devices Solutions for Use Therein, and Uses Thereof', issued Feb. 20, 1990 to H. J. Byker, which is incorporated herein in its entirety by reference, solutions of electrochromic compounds are described. These solutions are useful as the media of variable transmittance in electrochromic devices. The devices, in turn, are useful as the variable transmittance components in variable transmission light filters, such as windows, and variable reflectance mirrors, such as anti-glare rearview mirrors in automobiles.
Described in the aforementioned patent are components of the formula: ##STR1##
wherein R.sub.76 is oxygen or sulfur R.sub.80 is hydrogen or dialkylamino, wherein the alkyl groups are the same or different and are each of 1 to 6 carbon atoms, and R.sub.77 and R.sub.78 are the same or different and are each selected from hydrogen, alkyl of 1 to 6 carbon atoms, phenyl optionally substituted at any one position with an alkyl group of 1 to 6 carbon atoms, and benzyl, optionally substituted at any one position of the phenyl group with an alkyl group of 1 to 6 carbon atoms. Components of this formula generally have redox potentials below about 90 mV (using E.sub.1/2 (1) of 5,10-dimethyl-5,10-dihydrophenazine in propylene carbonate equal to 0.300V as an arbitrary reference), E.sub.SOMO -E.sub.HDOMO values below 3.6 eV, and dipole moment configurations that are long axis polarized. U.S. Pat. No. 4,802,108 teaches nothing of NIR absorption, nor does it teach that the electrochromic compounds disclosed therein, including those shown in compound LII, absorb in the NIR portion of the electromagnetic spectrum. Additionally, this patent teaches nothing of using the E.sub.SOMO -E.sub.HDOMO or the dipole moment configuration as criteria for identifying NIR absorbing compounds.
There has existed a need, then, to provide an electrochromic system that will provide reversibly variable transmittance in both the visible and near infrared portions of the solar spectrum. One device in which an electrochromic device acts as an adjustable solar energy barrier is disclosed in U.S. Pat. No. 5,239,406, entitled "Near-Infrared Reflecting, Ultraviolet Protected, Safety Protected, Electrochromic Vehicular Glazing, " N. R. Lynam discusses the use of the Cardinal Heat Mirror, which comprises a thin metal film, as a near IR reflector. In this example, three glass elements are used--two to make up the electrochromic cell and a third glass element that has the thin metal film and is attached to one side of the electrochromic cell. Such a design is complicated and expensive to manufacture. Moreover, the IR reflectance is permanent, and only the visible transmittance is "adjustable". Rather than having a permanent IR reflector, it would be desirable to enable modulation of near infrared radiation in order to take advantage of heat gain in winter. In an article titled "Electrochromic Devices: A Comparison Of Several Systems" Solar Energy Materials and Solar Cells, 39 (1995) 213-222, C. Arbizzani et al. discuss the use of poly(alkylthiophenes), poly(pyrroledodecylsulfate) and tungsten oxide in various combinations in variable light transmission electrochromic devices and suggest that these systems may be useful to regulate the solar energy flux to provide energy savings in buildings.
Devices based on these materials have not, to date, surpassed the use of devices based on solution-phase organic materials in the market, generally because of their lack of stability and high cost. It would be advantageous to have a solution-phase electrochromic compound that in its inactivated state would be colorless or nearly colorless and, when activated or colored, would absorb in the NIR (and preferably also in the visible) portion of the solar spectrum, and additionally have appropriate stability, cost and appearance to be aesthetically pleasing to a building occupant. However, an acceptable compound must also satisfy a number of other requirements. For example, the compound must be sufficiently soluble in electrochromic compositions, and must not interact with other components in the composition in a way that interferes with using the composition for its intended purpose. Thus, if the compound is to be used in a solution employed as a medium of variable transmittance to visible and NIR light in an electrochromic device, it must not interact with the other electrochromic compounds or other components of the medium in a way such that they cease to function effectively in the medium.
In addition to providing suitable NIR electrochromic compounds, it would also be advantageous to set forth physiochemical characteristics that one may use to identify new materials with near infrared absorbencies. Consequently, it is desirable to provide a set of criteria to identify electrochromic compounds that would be stable in an electrochromic medium as a part of an electrochromic device capable of reversibly changing its absorbance at wavelengths between 750 nm and 2400 nm. In addition, it is desirable to provide electrochromic devices using these NIR absorbing compounds.